The present invention relates to the determination of the effect aging has on mechanical properties and conditions of objects, e.g. specimens of plastic, synthetic, and other materials.
Methods are known according to which a specimen is alternatingly subjected to a load and relieved from that load. The load condition is carried out by deforming the object (specimen) in a particular manner. For example, the specimen is extended for a particular distance (stress load), or the specimen is subjected to torsion by twisting it by a particular angle, or the specimen is compressed, or subjected to shear force etc., and the reaction force in the specimen is measured. Generally speaking, one will test and work the material in a manner that may represent the dominating wear and load conditions of the specific subsequent use. On the other hand, the material may include or be subjected to forced aging and one wants to know the progress of that process.
Many plastic materials, polymers, elastomers, etc., can be tested in that manner and the differences in (elastic) reaction forces is indicative of aging. In between two tests, the specimen may be subjected to artificial aging processes such as heating, cooling, subjecting the specimens to gases, radiation, liquids, etc.
Particularly after such aging the measurments proceeded in a manner that the specimen is, e.g. alternatingly extended and relieved from the extension until the peak values of the reaction forces during a period of extension (loading) remained constant from each load-no-load cycle to the next one. In other words, one used an equilibrium condition as indication of the specific elastic state which, in turn, is to be indicative of aging. That equilibrium was, of course, dynamically obtained through the sequence of load-no-load cycles. Unfortunately, these equilibrium conditions do not occur rapidly but often after long periods of time up to an hour or even longer, and the material has to be worked through all this time by subjecting it to the sequence of load-no-load cycles. Aside from the operational (and economic) effort exerted therewith, long periods of working the specimens needed to attain the state of constant reaction force maxima from each load cycle phase to the next one, is a period in which the internal state of the specimen may change due to physical relaxation and/or chemical decomposing etc. Thus, the measurements purporting to represent aging may not be true on that account. The measurement is a composite of the effect of the discontinuous stress-relaxation working method and of other, parallel running, internal process. For this reason the equilibrium state may actually never be reached. This is particularly so if the specimen are subjected to aging fluids which remain in contact with the specimen during measurement; the aging process continues.
The German printed patent application P 24 47 624.2 discloses such a discontinuous stress-relaxation measurement and proposes to correct the specimen extension, e.g. by clamping the specimen anew after each load phase. This way, the actual extension exerted is the same from cycle to cycle, and one does not have to wait until a dynamic equilibrium with constant force peaks has been established. Thus, continuing aging and other internal processes will not falsify the measurement. However, the correcting operation is very cumbersome and expensive. One has to either clamp the specimen to the test equipment anew for each new load phase, or, as an alternative, one has to measure the needed length extension and provide the correction on that basis.
Another aspect is to be seen in the fact that frequently the first extension produces a much longer reaction than subsequent ones which is due to an ordering process in the material. This ordering process is actually superimposed and tends to introduce errors in the recognition of aging.